1. Field of the Invention
The present invention relates to an electrostatic latent image developing toner, and particularly to an electrostatic latent image developing toner which is excellent in low-temperature fixability and fixation separability, and is capable of yielding a toner image with an excellent high temperature offset resistance even on a rough paper having a large surface irregularity.
2. Description of Related Art
In a recent field of an electrostatic latent image developing toner (also simply referred to as “toner”, hereinafter), in order to satisfy market needs, an electrophotographic device suitable for the needs, and the toner usable for the electrophotographic device have been developed at a high pace. For example, a toner compatible to higher image quality is required to have a sharp particle size distribution. By equalizing the sizes of toner particles to sharpen the particle size distribution, behaviors of the individual toner particles are equalized in developing, resulting in distinctive improvement in reproducibility of fine dots. However, it has not been easy to sharpen the particle size distribution of the toner, by conventional toner manufacturing methods using a pulverization process.
As a countermeasure, there has been proposed an emulsion flocculation method, as a manufacturing method which can arbitrarily control shapes and a particle size distribution of the toner particle. This method is implemented by mixing an emulsified dispersion of resin particles with a colorant particle dispersion and an optional wax particle dispersion, allowing the individual particles to flocculate by adding a flocculant and/or controlling pH under stirring, and then fusing the particles under heating, to thereby obtain the toner particle.
Meanwhile, from the viewpoint of energy saving, development has been ongoing for a low temperature fixable toner which is fixable with less energy. In order to lower the fixing temperature of toner, it is necessary to lower the melting temperature and melt viscosity of binder resin. However, lowering a glass transition point and a molecular weight of the binder resin, aimed at lowering the melting temperature and melt viscosity of the binder resin, results in another problem of degrading the heat-resistant storability and fixation separability of the toner.
There has been reported a technique to control the toner particle so that it has a core-shell structure, aiming at appropriately balancing the low-temperature fixability and heat-resistant storability (see Japanese Patent Application Laid-Open Publication No. 2005-221933, for example). More specifically, the low-temperature fixability and heat-resistant storability can be well balanced, by forming, over a surface of a core particle with an excellent low-temperature fixability, a shell layer composed of a resin with a high softening point and an excellent heat-resistant storability. In particular, such shape control can be easily performed in manufacturing of the toner by the emulsion flocculation method.
As an example of the toner having the core-shell structure, there has been developed a toner using a polyester resin for the shell layer of the toner particle (see Japanese Patent Application Laid-Open Publication No. 2005-338548, for example). The polyester resin is advantageous in that it may be easily designed to lower the softening point, while keeping the glass transition point higher as compared with a styrene-acrylic resin. By thus using the polyester resin for the shell layer, a toner with excellent low-temperature fixability and heat-resistant storability is obtainable.
However, the styrene-acrylic resin has only a poor affinity to the polyester resin, so that, for the case where the styrene-acrylic resin is used for the core and the polyester resin is used for the shell layer, it has been difficult to form a thin and uniform shell layer, and thereby a sufficient level of the heat-resistant storability has not been achieved. Moreover, due to poor fusion between the core and the shell, the shape control of the toner particle has been difficult, and it has consequently been difficult to produce a dense and smooth toner particle in which the shell layer has a uniform surface. Due to a poor anti-crush performance, the shell layer may separate under toner agitation in a developing machine during successive printing, and as a consequence, the amount of electrical charge would largely fluctuate, and an image would have noise and would be degraded in quality.
To solve these problems, there has been proposed a toner having the core-shell structure in which a urethane-modified polyester resin or acryl-modified polyester resin is used for the shell layer (see Japanese Patent Application Laid-Open Publication No. 2005-173202, for example). Also disclosed is a technique to improve the low-temperature fixability, anti-offset performance, and temperature dependence of the electrical charge amount, by using, for the binder resin of the toner, a resin obtained by combining a polyester resin unit via a divalent crosslinking group (see Japanese Patent Application Laid-Open Publication No. 2011-28257, for example).
By using the urethane-modified polyester resin or the acryl-modified polyester resin as a resin composing the shell layer for the purpose of improving affinity between the styrene-acrylic resin and the polyester resin, the shell layer with a certain level of uniformity has been obtained, even if the core was configured by the styrene-acrylic resin. The shell layer, however, has an elevated glass transition point due to absence of a styrene component, and this damages the low-temperature fixability. Further efforts of enhancing the low-temperature fixability, such as lowering the softening point of the core resin to further give the low-temperature fixability, again resulted in the degraded fixation separability and high temperature offset resistance. Such method is therefore still insufficient to satisfy all of the low-temperature fixability, fixation separability and high temperature offset resistance.
Meanwhile, in a recent field of production printing, a copying machine and a printer have been directed to a higher operating speed and a wider range of paper types applicable thereto. Since the higher operating speed means a shorter time a transfer medium passes through a fixing unit, so that the toner has been required to be fixable with a smaller amount of energy. Also the toner has been required to be fixable on transfer media which have conventionally been used only with difficulty, such as cardboard, envelope, rough paper with a large surface irregularity and so forth. In this situation, even the core-shell type toner described above has been insufficient to satisfy all of the low-temperature fixability, fixation separability and high temperature offset resistance.
Moreover, the toner particle having the core-shell structure as described above has been known to have another problem that it could not fully exhibit the heat characteristic ascribable to the resin composing the core particle, due to the presence of the shell layer over the surface of the toner particle.